<Plug>(SneakStreak*) xmap, omap

- provide <Plug>(SneakStreak*) in visual-mode and operator-pending mode
- stub doc for sneak# API
- remove doc for deprecated commands
visual-mode: eliminate flicker

- buttery smooth (?)
- avoid need for <esc> with xnoremap
- remove spurious 'range' markers
- sneak#rpt(): use v:count1 instead of requiring parameter